Speed ratio control system of continuously variable transmission

Abstract

A control unit of a continuously variable transmission computes a speed ratio command value based on a final target ratio, a time constant representing a predetermined dynamic characteristic and a time constant representing an estimated dynamic characteristic of the transmission so that a real speed ratio approaches the final target ratio according to a running state under the predetermined dynamic characteristic. A transient target ratio is computed based on the final target ratio and time constant representing the predetermined dynamic characteristic, a correction amount of the speed ratio command value is computed based on the difference between the transient target ratio and real speed ratio, and the transmission is controlled based on the speed ratio command value after correction by this correction amount. In this way, the difference between the transient target ratio and real speed ratio is reduced and a desired speed ratio response is obtained even when the operation of a speed ratio mechanism is limited and the real speed ratio has shifted from the transient target ratio.

What is claimed is:  
     
       1. A speed ratio control system of a continuously variable transmission, comprising: 
       a first compensator which computes a speed ratio command value based on a final target ratio determined according to a running state, a time constant representing a predetermined dynamic characteristic and a time constant representing an estimated dynamic characteristic of the transmission,  
       a transient target ratio calculating element which computes a transient target ratio based on the final target ratio and the time constant representing the predetermined dynamic characteristic,  
       a correction amount calculating element which computes a correction amount of the speed ratio command value based on the difference between the transient target ratio and a real speed ratio, and  
       an adder/subtractor which corrects the speed ratio command value based on the correction amount, wherein:  
       the transmission is controlled based on the speed ratio command value after correction.  
     
     
       2. A speed ratio control system as defined in  claim 1 , comprising a second compensator which further corrects the speed ratio command value by a filter using approximate zeroing so that disturbances acting on the transmission are compensated. 
     
     
       3. A speed ratio control system as defined in  claim 2 , wherein the correction amount calculating element computes the correction amount of the speed ratio command value by passing the difference between the transient target ratio and real speed ratio through a function determined according to the time constant representing the dynamic characteristic of the transmission. 
     
     
       4. A speed ratio control system as defined in  claim 1 , wherein the correction amount calculating element computes the correction amount of the speed ratio command value by passing the difference between the transient target ratio and real speed ratio via a filter whereof the stability relative to variation of a time constant representing the dynamic characteristic of the transmission, is compensated. 
     
     
       5. A speed ratio control system as defined in  claim 4 , wherein the filter has an integrating characteristics. 
     
     
       6. A speed ratio control system as defined in  claim 4 , wherein the filter is designed so that the frequency characteristics of a closed loop control system comprising the correction amount computing element and the transmission have a gain of 1 in the low frequency region, and have no gain peak in the high frequency region. 
     
     
       7. A speed ratio control system as defined in  claim 4 , wherein the filter is designed using a generalized controlled system including the variation of the time constant representing the dynamic characteristic of the transmission. 
     
     
       8. A speed ratio control system as defined in  claim 4 , wherein the filter is designed using a generalized controlled system including a dead time of the dynamic characteristic of the transmission. 
     
     
       9. A speed ratio control system as defined in  claim 4 , wherein, when there is a dead time in the dynamic characteristic of the transmission, the transient target ratio calculating element computes the transient target ratio taking account of a lag corresponding to the dead time. 
     
     
       10. A speed ratio control system as defined in  claim 1 , wherein the first compensator computes the speed ratio command value by passing the final target ratio via a filter which is obtained by multiplying the inverse of a transmission function representing a dynamic characteristic of the transmission by a transmission function representing the predetermined dynamic characteristic. 
     
     
       11. A speed ratio control system as defined in  claim 1 , wherein the first compensator computes the speed ratio command value by passing the final target ratio via a first order/first order filter expressed by          G   TC     =           T   P     ·   s     +   1           T   T     ·   s     +   1                       
       T P =time constant representing a dynamic characteristic of the transmission and  
       T T =time constant representing the predetermined dynamic characteristic.  
     
     
       12. A speed ratio control system of a transmission, comprising a microprocessor programmed to: 
       compute a speed ratio command value based on a final target ratio determined according to a running state, a time constant representing a predetermined dynamic characteristic and a time constant representing an estimated dynamic characteristic of the transmission,  
       compute a transient target ratio based on the final target ratio and the time constant representing the predetermined dynamic characteristic,  
       compute a correction amount of the speed ratio command value based on the difference between the transient target ratio and real speed ratio, and  
       correct the speed ratio command value based on the correction amount, wherein:  
       the transmission is controlled based on the speed ratio command value after correction.  
     
     
       13. A speed ratio control system as defined in  claim 12 , wherein the microprocessor is further programmed to further correct the speed ratio command value by a filter using approximate zeroing to compensate for disturbances acting on the transmission. 
     
     
       14. A speed ratio control system as defined in  claim 12 , wherein the microprocessor is further programmed to compute a correction amount of the speed ratio command value by passing the difference between the transient target ratio and real speed ratio of via a filter whereof the stability relative to variation of a time constant representing a dynamic characteristic of the transmission, is compensated. 
     
     
       15. A speed ratio control system of a continuously variable transmission, comprising: 
       means for computing a speed ratio command value based on a final target ratio determined according to a running state, a time constant representing a predetermined dynamic characteristic and an estimated time constant representing a dynamic characteristic of the transmission,  
       means for computing the transient target ratio based on the final target ratio and the time constant representing the predetermined dynamic characteristic,  
       means for computing a correction amount of the speed ratio command value based on the difference between the transient target ratio and real speed ratio, and  
       means for correcting the speed ratio command value based on the correction amount, wherein:  
       the transmission is controlled based on the speed ratio command value after correction.  
     
     
       16. A speed ratio control system as defined in  claim 15 , further comprising means for correcting the speed ratio command value by a filter using approximate zeroing to correct for disturbances acting on the transmission. 
     
     
       17. A speed ratio control system as defined in  claim 15 , wherein the means for computing the correction amount of the speed ratio command value computes the correction amount of the speed ratio command value by passing the difference between the transient target ratio and real speed ratio via a filter whereof the stability relative to variation of a time constant representing the dynamic characteristic of the transmission, is compensated.